The present invention relates generally to the field of mobile satellite communication systems, and more particularly, to a power control method for controlling the transmit power of a mobile terminal in a satellite communication system.
Satellite systems are playing an increasing role in mobile communications by providing coverage in zones where land-based infrastructures are unable or ineffective to supply mobile services. Satellite systems can, for example, provide coverage in large, sparsely-populated areas where it is not cost effective to implement network infrastructures. Satellite systems can also make services available to airborne and sea-based users. Another use of satellite systems is to back-up or supplement a land-based network. Satellite systems can continue to provide services to users when segments of the network infrastructure are down, can carry additional traffic during peak transmission periods when the land-based network is overloaded, and can fill-in holes in the coverage area of a land-based system caused by man-made or natural obstacles.
Satellite communication systems are typically a derivative of GSM, which is adapted for use in a satellites system. The GSM technical specifications are published by the European Telecommunications Standard Institute (ETSI) and are incorporated in their entirety by reference herein. In GSM, the mobile terminal""s power level is controlled by the base station. The algorithm used by the base station is quite complex. To put it simply, the base station takes two measurements, called RXQUAL and RXLEV, based on the received signal. These measurements are used to determine the transmit power level for the mobile terminal. RXQUAL is based on bit error rates (BER) and RXLEV is based on the strength of the received signal. These measurements are generally taken over one complete SACCH period. Once the base station determines the appropriate power level for the mobile terminal, it sends a signaling message to the mobile terminal to tell it to adjust its transmit power level. The mobile unit has no part in deciding its transmit power level. While this method of power control works well in land-based cellular systems for which GSM was developed, it has shortcomings when applied in a mobile satellite communication system.
In a land-based cellular system, the propagation time for the transmission of a signal is relatively short. In a satellite system, the signal travels great distances before reaching its intended receiver. Consequently, there is a substantial propagation delay in satellite communications. The power control method used in GSM does not take into account these propagation delays. If, for example, the base station sends a message to the mobile terminal telling it to increase its power, the mobile station will not receive the message until approximately four SACCH cycles later. By that time the mobile station might be out of its disadvantaged location. The mobile unit would then transmit at a power level that is greater than the minimum needed to assure a desired signal quality which will result in an unnecessary drain on the mobile units battery. Conversely, if the base station commands the mobile terminal to reduce its transmit power because of favorable conditions, the designated transmit power level might be too low to maintain the desired signal quality by the time the mobile terminal receives the command.
Accordingly, there is a need for a new power control method that is specifically designed for use in a satellite system and takes into account the long propagation delays of a satellite system.
The present invention is a method for controlling the transmit power in a mobile radio communication device in a satellite communication system. The power control method of the present invention comprises both a closed-loop element and an open-loop element. For the closed-loop element, the base station calculates a power control constant for the mobile terminal based on the strength of signals received from the mobile terminal. The base station takes into account the propagation delays of the satellite system in determining the mobile terminal""s transmit power level. The mobile terminal uses the power control constant to calculate its transmit power so that changing the power control constant causes a corresponding increase or decrease in the mobile terminal""s transmit power.
The open-loop element of the power control method allows the mobile terminal to adjust its own transmit power level based on the strength of signals received from the base station while it waits for a new transmit power setting from the base station. It is assumed that the path loss on the uplink will be the same as the path loss on the downlink. If the power of the received signal diminishes on the down-link without any change in the transmit power level, it is assumed that the path loss is increasing. The mobile unit monitors the signal received from the base station and varies its transmit power level inversely in accordance with observed variations in the signal strength of signals received in the previous frame.
When discontinuous transmission (DTX) is employed in order to conserve power, transmissions are silenced in the direction not currently speaking. According to one aspect of the invention, one frame in every 13 TDMA frames always contains an active transmission of SACCH information even when DTX is used. The SACCH information is preferably transmitted at a constant power level to provide a stable signal strength reference point. The SACCH occurring one frame in thirteen does not allow fast control of the uplink power level since the 12 intervening frames receive no separate power control information. According to another aspect of the invention, the mobile terminal may receive a SACCH signal in a timeslot other than its allocated timeslot in order to determine if the path loss is varying. The alternate slot used for this purpose is preferably as close as possible in time to the mobile terminal""s assigned transmit slot without causing difficulty in switching from receive to transmit sufficiently rapidly.